1. Substitution for something not to be said out loud.
2. Substitution for something that cannot be thought of at the present moment.
3. Question, literally meaning either
i. "Are you still listening?"
or ii. "Do you think you understand?" (informal)

1. - You want me to ... you know ... ?
- What, suck me off?
- Yeah.
2. - So I decided to give her that thing.
- What thing?
- The thing, you know.
3. - She didn't like the thing, so she dumped me. You know?
- Yeah.